RDMA/rtrs-srv: Release lock before call into close_sess

In this error case, we don't need hold mutex to call close_sess.

Fixes: 9cb83748 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20201217141915.56989-4-jinpu.wang@cloud.ionos.com


Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Tested-by: Lutz Pogrell <lutz.pogrell@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>